3 Inheritance

8/2/2019 3 Inheritance

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Inheritance and Polymorphism


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Motivations

Suppose you will define classes to model circles,rectangles, and triangles. These classes have many

common features. What is the best way to design

these classes so to avoid redundancy? The answeris to use inheritance.


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Objectives To develop a subclass from a superclass through inheritance.

To invoke the superclasss constructors and methods using the super keyword.

To override instance methods in the subclass.

To distinguish differences between overriding and overloading.

To explore the toString() method in the Object class.

To discover polymorphism and dynamic binding.

To describe casting and explain why explicit downcasting is necessary.

To explore the equals() method in the Object class.

To store, retrieve, and manipulate objects in an ArrayList .

To implement a Stack class using ArrayList .

To restrict access to data and methods to subclasses using the protected visibilitymodifier.

To prevent class extending and method overriding using the final modifier.


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Superclasses and Subclasses

GeometricObject1

Circle4

Rectangle1

TestCircleRectangle

Run

GeometricObject

-color: String

-filled: boolean

-dateCreated: java.util.Date

+GeometricObject()

+GeometricObject(color: String,

filled: boolean)

+getColor(): String

+setColor(color: String): void

+isFilled(): boolean

+setFilled(filled: boolean): void

+getDateCreated(): java.util.Date

+toString(): String

The color of the object (default: white).

Indicates whether the object is filled with a color (default: false).

The date when the object was created.

Creates a GeometricObject.

Creates a GeometricObject with the specified color and filled

values.

Returns the color.

Sets a new color.

Returns the filled property.

Sets a new filled property.

Returns the dateCreated.

Returns a string representation of this object.

Circle

-radius: double

+Circle()

+Circle(radius: double)

+Circle(radius: double, color: String,

filled: boolean)

+getRadius(): double

+setRadius(radius: double): void

+getArea(): double

+getPerimeter(): double

+getDiameter(): double

+printCircle(): void

Rectangle

-width: double

-height: double

+Rectangle()

+Rectangle(width: double, height: double)

+Rectangle(width: double, height: double

color: String, filled: boolean)

+getWidth(): double

+setWidth(width: double): void

+getHeight(): double

+setHeight(height: double): void

+getArea(): double

+getPerimeter(): double


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Superclasss Constructor Is Always Invoked

A constructor may invoke an overloaded constructor or its

superclasss constructor. If none of them is invokedexplicitly, the compiler puts super() as the first statement

in the constructor. For example,

public A(double d) {

// some statements

}is equivalent to

public A(double d) {

super();

// some statements

}

public A() {

} is equivalent to public A() {super();}


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Using the Keyword super

To call a superclass constructor

To call a superclass method

The keyword super refers to the superclassof the class in which super appears. This

keyword can be used in two ways:


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CAUTION

You must use the keyword super to call the

superclass constructor. Invoking a

superclass constructors name in a subclasscauses a syntax error. Java requires that the

statement that uses the keyword super

appear first in the constructor.

C Ch i i


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Constructor Chaining

public class Faculty extends Employee {

public static void main(String[] args) {

new Faculty();}

public Faculty() {

System.out.println("(4) Faculty's no-arg constructor is invoked");

}

}

class Employee extends Person {

public Employee() {

this("(2) Invoke Employees overloaded constructor");

System.out.println("(3) Employee's no-arg constructor is invoked");

}

public Employee(String s) {

System.out.println(s);}

}

class Person {

public Person() {

System.out.println("(1) Person's no-arg constructor is invoked");

}

}

Constructing an instance of a class invokes all the superclasses constructors

along the inheritance chain. This is called constructor chaining.


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Trace Executionpublic class Faculty extends Employee {


new Faculty();

}

public Faculty() {


}

}


public Employee() {this("(2) Invoke Employees overloaded constructor");


}


System.out.println(s);

}}

class Person {

public Person() {


}

}

1. Start from the

main method

animation


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new Faculty();

}

public Faculty() {


}

}




}



}}

class Person {

public Person() {


}

}

2. Invoke Faculty

constructor

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

3. Invoke Employees no-

arg constructor

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

4. Invoke Employee(String)

constructor

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

5. Invoke Person() constructor

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

6. Execute println

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

7. Execute println

animation


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

8. Execute println

animation

i i


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new Faculty();

}

public Faculty() {


}

}




}



}

}

class Person {

public Person() {


}

}

9. Execute println

animation


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Example on the Impact of a Superclass

without no-arg Constructor

public class Apple extends Fruit {

}

class Fruit {

public Fruit(String name) {

System.out.println("Fruit's constructor is invoked");

}

}

Find out the errors in the program:


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Declaring a Subclass

A subclass extends properties and methods from thesuperclass. You can also:

Add new properties

Add new methods

Override the methods of the superclass


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Calling Superclass Methods

You could rewrite the printCircle() method in the Circle class as

follows:

public void printCircle() {

System.out.println("The circle is created " +super.getDateCreated() + " and the radius is " + radius);

}


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Overriding Methods in the Superclass

A subclass inherits methods from a superclass. Sometimes it is

necessary for the subclass to modify the implementation of a methoddefined in the superclass. This is referred to as method overriding.

public class Circle extends GeometricObject {

// Other methods are omitted

/** Override the toString method defined in GeometricObject */

public String toString() {

return super.toString() + "\nradius is " + radius;

}

}


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NOTE

An instance method can be overridden only

if it is accessible. Thus a private method

cannot be overridden, because it is not

accessible outside its own class. If a method

defined in a subclass is private in its

superclass, the two methods are completely

unrelated.


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NOTE

Like an instance method, a static method

can be inherited. However, a static method

cannot be overridden. If a static method

defined in the superclass is redefined in a

subclass, the method defined in the

superclass is hidden.


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Overriding vs. Overloading

public class Test {

public static voidmain(String[] args) {

A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {

public voidp(double i) {

System.out.println(i * 2);}

}

class A extends B {

// This method overrides the method in Bpublic voidp(double i) {

System.out.println(i);

}}

public class Test {

public static voidmain(String[] args) {

A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {

public voidp(double i) {

System.out.println(i * 2);}

}

class A extends B {

// This method overloads the method in Bpublic voidp(int i) {

System.out.println(i);

}}


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The Object Class and Its Methods

Every class in Java is descended from thejava.lang.Object class. If no inheritance is

specified when a class is defined, the

superclass of the class is Object.

public class Circle {

...

}

Equivalentpublic class Circle extends Object {

...}


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The toString() method in Object

The toString() method returns a string representation of the

object. The default implementation returns a string consistingof a class name of which the object is an instance, the at sign(@), and a number representing this object.

Loan loan = new Loan();

System.out.println(loan.toString());

The code displays something like Loan@15037e5 . Thismessage is not very helpful or informative. Usually you shouldoverride the toString method so that it returns a digestible stringrepresentation of the object.


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Polymorphism, Dynamic Binding and Generic Programmingpublic class PolymorphismDemo {


m(new GraduateStudent());

m(new Student());

m(new Person());

m(new Object());

}

public static void m(Object x) {

System.out.println(x.toString());

}

}

class GraduateStudent extends Student {

}

class Student extends Person {


return "Student";

}

}

class Person extends Object {


return "Person";

}

}

Method m takes a parameter

of the Object type. You can

invoke it with any object.

An object of a subtype can be used wherever its

supertype value is required. This feature is

known aspolymorphism.

When the method m(Object x) is executed, theargument xs toString method is invoked. xmay be an instance of GraduateStudent,Student, Person, or Object. ClassesGraduateStudent, Student, Person, and Object

have their own implementation of the toStringmethod. Which implementation is used will bedetermined dynamically by the Java VirtualMachine at runtime. This capability is knownas dynamic binding.

PolymorphismDemo

Run


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Method Matching vs. BindingMatching a method signature and binding a method

implementation are two issues. The compiler finds amatching method according to parameter type, numberof parameters, and order of the parameters atcompilation time. A method may be implemented in

several subclasses. The Java Virtual Machinedynamically binds the implementation of the method atruntime. See Review Questions 10.7 and 10.9.


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Generic Programmingpublic class PolymorphismDemo {


m(new GraduateStudent());

m(new Student());

m(new Person());

m(new Object());

}

public static void m(Object x) {

System.out.println(x.toString());

}

}

class GraduateStudent extends Student {

}

class Student extends Person {


return "Student";

}

}

class Person extends Object {


return "Person";

}

}

Polymorphism allows methods to be usedgenerically for a wide range of object

arguments. This is known as genericprogramming. If a methods parametertype is a superclass (e.g., Object), youmay pass an object to this method of anyof the parameters subclasses (e.g.,Student or String). When an object (e.g., a

Student object or a String object) is usedin the method, the particularimplementation of the method of theobject that is invoked (e.g., toString) isdetermined dynamically.


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Casting ObjectsYou have already used the casting operator to convert variables of

one primitive type to another. Casting can also be used to convert anobject of one class type to another within an inheritance hierarchy. In

the preceding section, the statement

m(new Student());

assigns the object new Student() to a parameter of the Object type.

This statement is equivalent to:

Object o = new Student();// Implicit casting

m(o);

The statement Object o = new Student(), known as

implicit casting, is legal because an instance of

Student is automatically an instance of Object.


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Why Casting Is Necessary?Suppose you want to assign the object reference o to a variable of the

Student type using the following statement:

Student b = o;

A compilation error would occur. Why does the statement Object o =

new Student() work and the statement Student b = odoesnt? This isbecause a Student object is always an instance of Object, but anObject is not necessarily an instance of Student. Even though you cansee that o is really a Student object, the compiler is not so clever toknow it. To tell the compiler that o is a Student object, use an explicit

casting. The syntax is similar to the one used for casting amongprimitive data types. Enclose the target object type in parentheses andplace it before the object to be cast, as follows:

Student b = (Student)o; // Explicit casting


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Casting from

Superclass to SubclassExplicit casting must be used when casting an

object from a superclass to a subclass. This type

of casting may not always succeed.

Apple x = (Apple)fruit;

Orange x = (Orange)fruit;


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The instanceof Operator

Use theinstanceof

operator to test whether an object is an instanceof a class:

Object myObject = new Circle();

... // Some lines of code

/** Perform casting if myObject is an instance of

Circle */if (myObject instanceof Circle) {

System.out.println("The circle diameter is " +

((Circle)myObject).getDiameter());

...

}


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TIP

To help understand casting, you may alsoconsider the analogy of fruit, apple, andorange with the Fruit class as the superclassfor Apple and Orange. An apple is a fruit, so

you can always safely assign an instance ofApple to a variable for Fruit. However, afruit is not necessarily an apple, so you have

to use explicit casting to assign an instanceof Fruit to a variable of Apple.


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Example: Demonstrating

Polymorphism and CastingThis example creates two geometric objects: a

circle, and a rectangle, invokes the

displayGeometricObject method to display theobjects. The displayGeometricObject displays

the area and diameter if the object is a circle, and

displays area if the object is a rectangle.

TestPolymorphismCasting Run


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The equals Method

The equals() method compares the

contents of two objects. The default implementation of theequals method in the Object class is as follows:

public boolean equals(Object obj) {

return (this == obj);}

For example, the

equals method is

overridden in

the Circle

class.

public boolean equals(Object o) {

if (o instanceof Circle) {

return radius == ((Circle)o).radius;

}

else

return false;

}


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NOTEThe == comparison operator is used for

comparing two primitive data type values or fordetermining whether two objects have the same

references. The equals method is intended to

test whether two objects have the samecontents, provided that the method is modified

in the defining class of the objects. The ==

operator is stronger than the equals method, inthat the == operator checks whether the two

reference variables refer to the same object.


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The ArrayList and Vector Classes

You can create an array to store objects. But the arrays size is fixed

once the array is created. Java provides the ArrayList class that canbe used to store an unlimited number of objects.

java.util.ArrayList

+ArrayList()

+add(o: Object) : void

+add(index: int, o: Object) : void

+clear(): void

+contains(o: Object): boolean

+get(index: int) : Object

+indexOf(o: Object) : int

+isEmpty(): boolean

+lastIndexOf(o: Object) : int

+remove(o: Object): boolean

+size(): int

+remove(index: int) : Object

+set(index: int, o: Object) : Object

Appends a new element o at the end of this list.

Adds a new element o at the specified index in this list.

Removes all the elements from this list.

Returns true if this list contains the element o.

Returns the element from this list at the specified index.

Returns the index of the first matching element in this list.

Returns true if this list contains no elements.

Returns the index of the last matching element in this list.

Removes the element o from this list.

Returns the number of elements in this list.

Removes the element at the specified index.

Sets the element at the specified index.

Creates an empty list.


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Compile Warning

You will get a compilation warning unchecked

operation. Ignore it. This warning can be fixedusing generic types.

TestArrayList Run


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The MyStack Classes

A stack to hold objects.

MyStack

MyStack

-list: ArrayList+isEmpty(): boolean

+getSize(): int

+peek(): Object

+pop(): Object+push(o: Object): void

+search(o: Object): int

Returns true if this stack is empty.

Returns the number of elements in this stack.

Returns the top element in this stack.

Returns and removes the top element in this stack.Adds a new element to the top of this stack.

Returns the position of the first element in the stack from

the top that matches the specified element.

A list to store elements.


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The protected Modifier

The protected modifier can be applied on dataand methods in a class. A protected data or aprotected method in a public class can be accessedby any class in the same package or its subclasses,

even if the subclasses are in a different package.private, default, protected, public

private, none (if no modifier is used), protected, public

Visibility increases


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Accessibility Summary

Modifier

on members

in a class

Accessed

from the

same class

Accessed

from the

same package

Accessed

from a

subclass

Accessed

from a different

package

public

protected -

default - -

private - - -


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Visibility Modifiers

public class C1 {

public int x;

protected int y;

int z;

private int u;

protected void m() {

}

}

public class C2 {

C1 o = new C1();

can access o.x;

can access o.y;

can access o.z;

cannot access o.u;

can invoke o.m();

}

public class C3

extends C1 {

can access x;

can access y;can access z;

cannot access u;

can invoke m();

}

package p1;

public class C4

extends C1 {

can access x;

can access y;cannot access z;

cannot access u;

can invoke m();

}

package p2;

public class C5 {

C1 o = new C1();

can access o.x;

cannot access o.y;cannot access o.z;

cannot access o.u;

cannot invoke o.m();

}


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A Subclass Cannot Weaken the Accessibility

A subclass may override a protectedmethod in its superclass and change its

visibility to public. However, a subclass

cannot weaken the accessibility of amethod defined in the superclass. For

example, if a method is defined as public

in the superclass, it must be defined aspublic in the subclass.


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NOTE

The modifiers are used on classes andclass members (data and methods), except

that the final modifier can also be used on

local variables in a method. A final localvariable is a constant inside a method.


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The final Modifier

The final class cannot be extended:final class Math {

...

}

The final variable is a constant:

final static double PI = 3.14159;

The final method cannot beoverridden by its subclasses.

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